Although not widely practiced, the use of radiation energy transmitted by a fiberoptic light cable for the detonation of explosives is widely known.
Electrically actuated detonators comprising a thermo resistive element coated with an initiation or "flashing" compound are well known. Such devices are inexpensive to manufacture and are known to be extremely reliable in use with a reasonable safety factor in storage and handling within defined parameters.
A major difficulty experienced with electrically actuated detonators is that they require connection to the detonating device via electrical conductive cables. Accidential explosions have been attributed to spurious electrical currents induced or conducted in the electrical cables by electrical machinery, static earth charges, lightning and the like.
In an endeavour to avoid the use of electrically conductive cables between a detonator and a device supplying energy thereto for actuation, nonelectrically conductive fiberoptic cables have been proposed.
Detonators employing fiberoptic energy transmitters all suffer from two major disadvantages--high cost and unreliability.
U.S. Pat. No. 3,408,937 describes a laser actuated detonator having a fiberoptic cable terminating within a mass of pyrotechnic material or separated therefrom by a concentrating lens. In the former case laser energy emitting from the end of the fiberoptic cable impinges directly on the pyrotechnic material. As the pyrotechnic material is inherently a poor heat conductor, immense energy pulses are required to ignite the pyrotechnic material. It has been found that such immense energy pulses can simply blast through the pyrotechnic material and as the duration of the pulse is so short, the heat energy is dissipated in the pyrotechnic without causing ignition.
In the latter case the lens is considered to be ineffective as there is no optical pathway to focus the laser light when the pyrotechnic material is located against the face of the lens.
The detonator described in U.S. Pat. No. 3,408,937 requires an immense amount of laser energy to produce an ignition temperature of around 2000.degree. C. in the pyrotechnic material, but this amount of energy in a short duration pulse is sufficient to set up a shock wave capable of penetrating the pyrotechnic material without ignition. In a time delay detonator this can cause undesirable instant detonation of the explosive material in the detonator body.
U.S. Pat. No. 4,403,143 describes a detonating cord having a fiberoptic core formed therein. A free end of the fiberoptic cord is fitted with a reflector to enable continuity of the detonating cord to be checked. The detonating cord is ignited by a conventional electrically actuated detonator.
U.S. Pat. No. 4,391,195 describes a laser energy actuated detonator employing fiberoptic cable to transmit the laser energy. The detonator described in this patent comprises a sensitive pyrotechnic material coated on the end of a fiberoptic cable adjacent the detonator charge compound or in direct contact therewith in a manner similar to the detonator described in U.S. Pat. No. 3,408,937 above.
The problems of poor heat absorption in the pyrotechnic material are addressed by incorporating an energy absorbing pigment or dyestuff with a highly sensitive pyrotechnic.
U.S. Pat. No. 3,812,783 describes a laser actuated detonator comprising a focusing lens which focuses the laser energy on a metal film located on the opposite side of a transparent window. The laser energy causes the metal film to vaporize and cause a shock wave to detonate explosive material in the detonator body.
U.S. Pat. No. 3,747,530 recognizes the problems associated with energy losses and unreliability of transmission of laser energy through transparent windows associated with optical fibers.
None of the prior art laser actuated detonators have been successful in combining the three distinctive features of prior art electrically actuated thermo resistive detonators in low cost, reliability and safety.